High carbon steel sheets used, for example, for tools and automobile parts (gears and transmissions) are processed by heat treatment such as quenching and tempering after punching and/or molding. In recent years, in manufactures of tools and parts, that is, in customers of high carbon steel sheets, in order to reduce the cost, instead of part fabrication by cutting and hot forging of casting materials which has been performed in the past, simplification of fabrication steps has been studied by press molding (including cold forging) of steel sheets. Concomitant with this study, besides excellent quenching performance, a high carbon steel sheet as a raw material has been desired to have good workability so that a complicated shape is formed by a small number of steps and, in particular, has been strongly desired to have soft properties. In addition, in view of load decrease of pressing machines and metal molds, the soft properties are also strongly anticipated.
In consideration of the current situations, as for softening of a high carbon steel sheet, various techniques have been studied. For example, in Patent Document 1, a method for manufacturing a high carbon steel strip has been proposed in which after hot rolling, a steel strip is heated to a ferrite-austenite two phase region, followed by annealing at a predetermined cooling rate. According to this technique, a high carbon steel strip is annealed at the Ac1 point or more in the ferrite-austenite two phase region, so that a texture is formed in which rough large spheroidizing cementite is uniformly distributed in a ferrite matrix. In particular, after high carbon steel containing 0.2% to 0.8% of C, 0.03% to 0.30% of Si, 0.20% to 1.50% of Mn, 0.01% to 0.10% of sol. Al, and 0.0020% to 0.0100% of N, and having a ratio of the sol. Al to N of 5 to 10, is processed by hot rolling, pickling, and descaling, annealing is performed at a temperature range of 680° C. or more, a heating rate Tv (° C./Hr) in the range of 500×(0.01−N(%) as AlN) to 2,000×(0.1−N(%) as AlN), and a soaking temperature TA (° C.) in the range of the Ac1 point to 222×C(%)2−411×C(%)+912 for a soaking heating time of 1 to 20 hours in a furnace containing not less than 95 percent by volume of hydrogen and nitrogen as the balance, followed by cooling to room temperature at a cooling rate of 100° C./Hr or less.
For example, in Patent Document 2, a manufacturing method has been disclosed in which a hot-rolled steel sheet containing 0.1 to 0.8 mass percent of carbon and 0.01 mass percent or less of sulfur is sequentially processed by a first heating step at a temperature range of Ac1−50° C. to less than Ac1 for a hold time of 0.5 hours or more, a second heating step at a temperature range of Ac1 to Ac1+100° C. for a hold time of 0.5 to 20 hours, and a third heating step at a temperature range of Ar1−50° C. to Ar1 for a hold time of 2 to 20 hours, and in which the cooling rate from the hold temperature in the second step to that in the third step is set to 5 to 30° C./Hr. By performing the three-stage annealing as described above, it is attempted to obtain a high carbon steel sheet having an average ferrite grain diameter of 20 μm or more.
In addition, in Patent Documents 3 and 4, a method has been disclosed in which carbon contained in steel is graphitized so as to obtain softened steel having high ductility.
Furthermore, in Patent Document 5, a method for uniformly forming rough large ferrite grains to obtain ultra soft steel has been disclosed in which steel containing 0.2 to 0.7 mass percent of carbon is hot-rolled to control the texture so as to have more than 70 percent by volume of bainite, followed by annealing. According to this technique, after finish rolling is performed at a temperature of (the Ar3 transformation point−20° C.) or more, cooling is performed to a cooling stop temperature of 550° C. or less at a cooling rate of more than 120° C./sec, and after coiling at a temperature of 500° C. or less and pickling are performed, annealing is performed at a temperature in the range of from 640° C. to the Ac1 transformation point.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 9-157758
Patent Document 2: Japanese Unexamined Patent Application Publication No. 11-80884
Patent Document 3: Japanese Unexamined Patent Application Publication No. 64-25946
Patent Document 4: Japanese Unexamined Patent Application Publication No. 8-246051
Patent Document 5: Japanese Unexamined Patent Application Publication No. 2003-73742